1. Field of the Invention
The present invention relates to a radiation detecting device and a radiation detecting method, and more particularly to a radiation detecting device and a radiation detecting method which are suitable for use in detecting information such as images by converting the wavelength of radiation including X-rays by means of a wavelength converter typified by a scintillator (or phosphor) into a wavelength in a wavelength region detectable by a sensor element.
2. Related Background Art
When radiation such as an X-ray is detected directly by a photosensor in radiation diagnostic apparatus and X-ray photographic apparatus making use of an X-ray or the like, the efficiency of such an apparatus becomes poor because there is no photosensor having high sensitivity to the radiation. It is therefore considered to use a scintillator capable of converting the radiation into visible light and a photosensor in combination.
As the characteristics of the scintillator, there are characteristics called the afterglow characteristics of a luminescent screen. It is indicated that the light emission of a scintillator, which is attendant on radiation exposure, is caused and attenuated in a certain functional relation as illustrated in FIG. 1, and a slow component has a time constant as long as several hundreds milliseconds. In order to correct the attenuation of afterglow as a countermeasure thereof, in U.S. Pat. No. 5,331,682 by way of example, a great number of signal samples are detected to calculate out a compensation value by complicated calculation, and the compensation value is subtracted from the signals. In addition, for this calculation, delay is caused until the initial attenuation component can be neglected.
On the other hand, it is proposed in, for example, U.S. Pat. No. 5,262,649 to use a photosensor composed of thin film semiconductors in combination with a scintillator for X-ray photographic apparatus and radiation diagnostic apparatus making use of an X-ray or the like. In this publication, a relationship among the time constant according to the sensor composed of the thin film semiconductor, and a transistor, the read rate of the apparatus, and an S/N (signal/noise) ratio is described. In U.S. Pat. No. 5,262,649, there are introduced a reading method of a fluoroscopic mode in which an X-ray is continuously emitted, and a photographic mode in which an X-ray is emitted only for a short period of time, and all the sensors store signals at the same time.
However, in order to detect a great number of signals to calculate out a compensation value, and to make the calculation that the compensation value is subtracted from the signals as described in, for example, U.S. Pat. No. 5,331,682, expensive signal processing circuit and arithmetic unit are required. In addition, since delay is caused until the initial attenuation component can be neglected, the fetch of signals from a detector requires to wait by the delay time.
In U.S. Pat. No. 5,262,649, the fluoroscopic mode in which an X-ray is continuously emitted, and the photographic mode in which an X-ray is emitted only for a short period of time are introduced. In the photographic mode, no time constant of the light emission and attenuation of the scintillator is considered. Therefore, when reading is started in a moment after the irradiation with the X-ray is completed, due to the time constant of the light attenuation of the scintillator, a signal is read out in the initial line of the reading while a dark current is high, and a signal with a dark current component integrated is read out in a reading line on the last side. Therefore, the dark current mixed into the signal due to the delayed attenuation characteristics of the scintillator greatly varies according to the reading order of the line.
In U.S. Pat. No. 5,262,649, there is introduced an X-ray diagnostic apparatus or radiation therapeutic apparatus using a large screen sensor panel comprised of sensors composed of a-Si:H (amorphous silicon hydride) and thin film transistors, and a relationship among a time constant obtained by multiplying the capacity of the sensor by the ON resistance of the thin film transistor, an S/N ratio and a frame frequency is derived, which is required as a real-time image sensor. However, this relationship is on the assumption that an X-ray is continuously emitted, and such attenuation characteristics of the scintillator as described above are not referred to. This publication does also not refer to the design of reading when an X-ray is intermittently emitted.
The attenuation characteristics of the scintillator do not become a considerable problem in the case of the photographic mode or the like because there is sufficient time. In the case of a full moving image having many frames as in the diagnosis of the circulatory organ system, however, it is considered that the residual component of light may exert an influence as noise.
In such a case, however, it is not proposed to make a design by combining the attenuation characteristics of the scintillator with the reading characteristics of the time constant composed of the capacity of the sensor and the ON resistor of the thin film transistor in the sensor panel in such a case.